Optical data transmission over light transmitting fiber, continues to be an attractive alternative to other forms of information transfer. Optical fiber has a higher bandwidth and lower signal degradation over the same conductor length than the electrical counterpart. Compactness and immunity to electromagnetic interference renders optical fiber amenable to many types of operating environments.
Many optical connectors use adhesives to bond the fiber to the termination. Fiber optic cable consists of layers of different materials concentric about a fiber. A typical cable consists of a central glass fiber, a coating, a buffer, strands of strength members, and a jacket. As is commonly the case, the fiber is adherent to the coating, but the coating is not adherent to the buffer. Adhesives provide distributed retention of a fiber to the termination by providing a continuous restraint along the length of the termination adhered to the fiber and the buffered fiber. This distributed retention prevents slippage between the fiber and the buffer. Adhesives also provide stability of fiber retention over temperature and time.
Adhesives typically require either heating, a curing time, or both in order to create the desired bond. Many optical termination installations occur in the field and after an optical cable network is in place. Assembly of a termination using a thermoset adhesive requires heating equipment and power. Installation of each termination requires a warm up time for the heating unit and a curing time to set the adhesive bond. Assembly of terminations in many locations, therefore, requires an installer for each termination to (1) move the heating equipment and power, if necessary, to each termination location, (2) wait the warm-up time, (3) assemble the connector, (4) wait the curing time, (5) disassemble the equipment and (6) move to the next location. Systems having instant handling strength can be used either without an adhesive, the mechanical system providing all fiber retention, or in conjunction with an adhesive, the mechanical system providing sufficient retention of the fiber to permit an adhesive to cure over time without operator involvement. Systems having instant handling strength, therefore, can save a great deal of time and effort by obviating the need for heating equipment and power.
U.S. Pat. No. 5,088,804 to Grinderslev discloses a system that terminates a fiber optic cable mechanically and provides instant handling strength. Once the mechanical system is actuated, the fiber is aligned and retained. The mechanical system disclosed in the Grinderslev patent, uses three resilient spheres housed within a hollow alignment ferrule. A hollow plunger and an aperture created by the juxtaposition of the three spheres receives a fiber. The plunger forces the three spheres into a constriction in the alignment ferrule. The resulting deformation of the three spheres causes the aperture to contract and grip the fiber.
Mechanical fiber retention systems provide point sources of force loading for fiber retention. Due to the greater loading of the point source as opposed to the distributed retention system of adhesives, there is a greater risk of slippage between the fiber and the buffer. Slippage between fiber and buffer creates a difference in position between the physical end of the fiber and the physical end of the termination. Should slippage occur, an air gap would be present upon mating two fiber optic terminations causing degradation of the signal.
In mechanical detention systems, it is important that the bullet that grips the fiber be under constant compression over tile and temperature in order to maintain the integrity of the grip. The plunger effects bullet compression, therefore motionless plunger retention maintains the integrity of the grip. U.S. Pat. No. 5,080,460 to Erdman et al. discloses a plunger having annular barbs and a ferrule having corresponding ribs. The barbs frictionally engage the ribs to create an interference fit between the plunger and the ferrule that resists rearward movement of the plunger relative to the ferrule. The multiple ribs of the plunger provide multiple retention points that are beneficial characteristics of a distributed retention system.
Mechanical retention systems also provide multiple cuts on a rear of the plunger that create a collet. Upon engagement of the plunger with the ferrule assembly, the tines of the collet collapse inwardly and grip the buffer. The retention of the buffer by the collet provides an additional retention point that further distributes any load on the termination and resists slippage between the buffer and the fiber. The cuts that create the tines of the collet are wide relative to the fiber diameter. During assembly, the fiber can easily become entangled with the tines which can slow the assembly process. In addition, the manufacturing cost of producing the tines tends to be disproportionately high as compared to the cost of the entire termination.